1. Field of the Invention
The present invention relates to a screw pump containing a screw array comprising a central screw with convex flanks and one or more side screws with concave flanks, in which the threads have a form such that coacting screws seal against each other simultaneously as all screws are sealingly contained in a housing.
2. Description of the Prior Art
When such a pump is in operation, liquid is transported axially from the inlet or suction side of the screw array to its outlet or pressure side in closed chambers, which are formed by the flanks and roots of the screws and the surrounding housing. Such a chamber is formed at the suction side of the screw array, when the driving screw is rotated, and filled with liquid as it is formed. When the chamber is completely formed, it is closed on the suction side and continues, for continued rotation of the driving screw, axially towards the outlet side of the screw array where it opens and the liquid is discharged. The volume of the chamber is unaltered during the whole transport from inlet to outlet side, and providing the pump is ideally sealed, the liquid in the chamber is at the pressure prevailing at the inlet during the whole of this transport, and not until it arrives at the outlet is it given the higher pressure prevailing there. In a practical pump of this kind there is always some interior leakage of course, due to unavoidable manufacturing tolerances, which result in a given clearance between the screws themselves and between the screws and the housing, and there is thus a small pressure increase during transport through the screw array. With sufficient accuracy in manufacture this pressure increase will be so little, however, that substantially the whole of the pressure increase takes place at the outlet.
A sub-pressure occurs when the chamber is formed, resulting in that it is filled with liquid. If rotation takes place at a rate falling below a given critical value, the chamber is filled completely with liquid. This critical value depends on the geometrical dimensions and implementation of the screws, the configuration of the inlet, the properties of the liquid pumped, e.g. its viscosity, pressure of vaporization and its content of dissolved gases, and the suction height. If the revolutional rate exceeds the critical value there is no time for the chamber to be filled entirely, and cavities are formed in the liquid which are filled with gas or air dissipated by the pumped liquid. These gas or air bubbles and the air bubbles which the liquid contains when it is fed into the pump accompany the liquid during its transport in the screw array from the inlet to the outlet side. When the chamber is opened to the outlet side and the liquid is suddenly subjected to the pressure prevailing there, the gas and air bubbles will be rapidly compressed, i.e. they implode. This results in noise and vibrations which can be very disturbing and can also cause damage to the pump and other components in the installation.
In order to reduce these effects of cavitation so that it will be possible to drive the pump at a rate of revolutions exceeding the critical rate at which cavities normally occur, it has been arranged that each chamber during its migration is in communication with the pressure side of the pump via a narrow duct, such that the pressure in each individual chamber during transport from the inlet to the outlet side is continuously raised from the value at the inlet side to the value at the outlet side (see the French Pat. No. 1245463). The result of this is that the gas and air bubbles are successively compressed, thereby avoiding the sudden implosion of bubbles at the outlet side.
It has also been attempted to reduce the mentioned effects of cavitation by making a groove in at least one of the flanks of one of the screws, this groove forming a helical duct throughout the screw array, and when the groove is made in the flank of a side screw, it is within the transition between the concave flank and the cylindrical circumferential surface of the thread, and when it is made in the central screw outside the transition between the concave flank and the cylindrical bottom surface of the thread (see the Swedish Pat. No. 199 274).
A disadvantage with these previously known embodiments is that return leakage in the pump increases, and the increase in revolutional rate which is dependent on the size of the duct thus takes place at the expense of sealing. Furthermore, return leakage takes place continuously or successively mainly along the whole length of the screw array, which adversely affects efficiency. Another disadvantage is that the amount of liquid which is supplied to the chambers by return leakage or by liquid supply via the duct cannot be adjusted to different operational conditions, resulting in that the pump most often operates with poor efficiency. A further disadvantage is that providing the duct makes manufacture of the otherwise complicated screw even more complicated.